In recent years, lithium ion batteries and other sealed secondary batteries have gained ever more importance as power sources, for instance as vehicle power sources, and as power sources in personal computers, mobile terminals and the like. Lithium ion batteries, in particular, are lightweight and afford high energy densities, and are thus expected to become preferred high-output power sources installed in vehicles.
Such sealed batteries are provided with an internal pressure releasing mechanism for releasing internal pressure (gas pressure) in case that the gas pressure rises excessively, ordinarily on account of overcharge or the like. In a typical example, for instance such as the one disclosed in Patent document 1, the internal pressure release mechanism may be a safety valve configured in such a manner that a thin portion (typically, made of metal) that is thinner than other portions is formed in part of the case, such gas in the case is released through breakage (splitting) of the thin portion when the internal pressure in the case becomes equal to or higher than a predetermined value (release pressure), to lower thereby the internal pressure in the case. Patent documents 2 and 3 are other prior art documents relating to batteries that are provided with such safety valves.